1. Field of the Invention
The present invention relates to a sheet processing apparatus, which is provided, for example, in an apparatus main body of an image forming apparatus such as a copying machine or a printer, and applies processing to sheets to be sent from the apparatus main body. In particular, the present invention relates to a sheet processing apparatus, which can store sheets to be sent while processing is applied to the sheets, and an image forming apparatus including the sheet processing apparatus.
2. Related Background Art
In recent years, a sheet processing apparatus such as a sorter for sorting sheets, on which an image has been formed, as an option for an image forming apparatus such as an electrophotographic copying machine or a laser beam printer. This kind of sheet processing apparatus is adapted to apply one of sort processing, stitch processing, alignment processing, and the like to sheets.
For example, a sheet processing apparatus including a stapler for stitching sheets with needles is adapted to, after causing sheets, which are conveyed into a sheet processing apparatus main body, to pass through a conveyance path formed in the inside of the main body and stacking the sheets on a processing tray, perform a stitching action.
A sheet processing apparatus for stitching a sheet stack is adapted to stack sheets on a processing tray in bundles and move a stapler serving as stitching means to perform one position stitch or multiple-position stitch (usually two-position stitch). While a stitching action is performed, sheets of the next job cannot be stacked on the processing tray. Consequently, sheets are required to be supplied on the basis of job unit in which the stitching action is performed.
In a sheet processing apparatus which performs stitch processing other than the needle stitch processing, sheets are required to be supplied at intervals on the basis of job unit while the processing is applied to the sheets.
However, when the sheets are supplied at intervals, productivity declines. In other words, the number of sheets to be processed per unit time decreases. As a sheet processing apparatus for preventing the decline in productivity, there is a sheet processing apparatus which includes a sheet holding portion (buffer portion) for storing to cause sheets to stand by in a conveyance path in the course of conveyance of the sheets to a processing tray.
This sheet processing apparatus is adapted to, while processing is applied to plural sheets stacked on the processing tray, store subsequent plural sheets in the sheet holding portion and, at the point when the processing ends, stack the sheets stored in the sheet holding portion on the processing tray and supply the subsequent sheets to the processing tray until the sheets on the processing tray reach a desired number (e.g., see Japanese Patent Application Laid-Open No. H9-48545).
A conventional sheet processing apparatus 10 shown in FIG. 46 includes a buffer roller path 14, which winds sheets around a rotating buffer roller 13 to cause the sheets to stand by for conveyance to a post-processing tray 11, in a conveyance path 12 in the course of conveyance of the sheets to the post-processing tray 11.
With such a structure, the conventional sheet processing apparatus 10 stores sheets, which are conveyed from a discharge roller pair 17 in an apparatus main body 16 of an image forming apparatus 15, in the buffer roller path 14. After a preceding sheet stack has undergone, for example, a stitch action on the post-processing tray 11, and an upper roller 18a and a lower roller 18b of an oscillation roller pair 18 have nipped to discharge sheets, while rotating, from the post-processing tray 11, the sheet processing apparatus 10 conveys the sheet stack stored in the buffer roller 13 to the post-processing tray 11 to thereby prevent the decline in productivity without increasing conveyance intervals among the sheets during the stitch action.
However, since the conventional sheet processing apparatus 10 includes the buffer roller path 14 and requires a space for setting the buffer roller 13 and the buffer roller path 14, which stop conveyance of subsequent sheets to the post-processing tray 11 to cause sheets to stand by during a stitch action, a size of the sheet processing apparatus itself increases to cause an increase in costs.
In addition, since the conventional sheet processing apparatus 10 discharges sheets with the oscillation roller pair 18, a discharge action of sheets is irregular to cause unevenness of time required for sheet discharge.
Moreover, although the conventional sheet processing apparatus 10 is adapted to stack sheets, which are stored in the buffer roller path, on the post-processing tray 11 after discharging sheets on the post-processing tray 11, the sheet processing apparatus 10 is not suitable for the recent actual situation in which high-speed processing is required. Thus, an apparatus with shorter processing time has been expected.
In addition, in the sheet processing apparatus, the number of sheets to be stored in the sheet holding portion is fixed regardless of time required for processing sheets. For example, in the case of a sheet processing apparatus for stitching sheets, as the number of positions to be stitched increases, longer time is required for the processing. Thus, sheets of a number corresponding to longest required time for processing are stored in the sheet holding portion. Consequently, in the sheet processing apparatus for stitching sheets, in the case in which there are a small number of positions to be stitched, the sheet holding portion continues an action for storing sheets regardless of the fact that the processing has ended, and sheet processing efficiency is low. The sheet processing efficiency is also low in sheet processing apparatuses which perform other sheet processing.